(1) Field of the Invention
The present invention relates to a polymer dispersion type liquid crystal display element of light-scattering mode, a producing method therefor and an apparatus for use in the producing method. More particularly, the invention relates to a polymer dispersion type liquid display element for use in liquid crystal televisions, computer terminals, liquid crystal projectors, and others, a producing method therefor and an apparatus for use in the producing method.
(2) Description of the Prior Art
Liquid crystal display element are widely used in various kinds of articles, including displays of wristwatches, and electronic calculators, navigation systems, notebook PCs, liquid crystal monitors, data projectors and projection liquid crystal TVs, taking advantage of their characteristics of low-profile, compact size, low voltage drive, and low power consumption. Among display modes of the liquid crystal display element, the TN (Twisted Nematic) mode is widely used, in which a liquid crystal display element having a structure of liquid crystal molecules being twisted at 90xc2x0 below and above between two opposing, upper and lower substrates is held by two polarizers. The STN (Super Twisted Nematic) mode of liquid crystal display element designed to improve a time-division drive characteristic of the TN mode is also wisely used in, for instance, word processors for Japanese characters. Recently, information equipment is in actual use using ferro-electric liquid crystals, in which liquid crystal molecules are changed in alignment by means of spontaneous polarization of the liquid crystal molecules and the resultant electro-optical effects are used for displays.
These liquid crystal display element, however, require at least one polarizer, so that they involve the problems of darkness, necessity of alignment phase treatment and difficulties in controlling cell gap.
On the other hand, a different mode of liquid crystal display element has been proposed according to which the polarizer is no longer required for the liquid crystal display element, and the liquid crystal molecular alignments are controlled by electric field to produce an opaque state or a transparent state. In this mode of liquid crystal display element, a complex of liquid crystals and transparent polymers is held between two substrates and an ordinary ray refractive index of the liquid crystal molecules with anisotropy in positive dielectric constant is adapted to match with a refractive index of the transparent polymers. When a voltage is applied to allow the major axes of the liquid crystal molecules to be aligned in parallel with the electric field, to match the refractive index of the liquid crystal molecules with refractive index of the transparent polymers, the scattering of light does not occur at the phase boundary to thereby produce the transparent state. On the other hand, while no voltage is applied thereto, the liquid crystal molecules are being oriented in different directions and their refractive index does not match with that of the transparent polymer at the phase boundary between the liquid crystal and the transparent polymer, so that the light scattering occurs to produce the opaque state. With the aid of this phenomenon, the opaque state or the transparent state is produced.
A representative example of this mode is what is called NCAP (Nematic Curvilinear Aligned Phase) in which nematic liquid crystals are micro-encapsulated with polyvinyl alcohol or equivalent (Fine Particles and Industry, Vol. 22, No. 8 (1990)).
Further, there is another mode called PDLC (Polymer Dispersed Liquid Crystals) in which liquid crystal fine droplets are dispersed in polymer matrix (Flat Panel Display ""91, on page 219, published by NIKKEI BP).
Also, there is a still another mode called PNLC (Polymer Network Liquid Crystals) having a structure of resins spreading in a continuous phase of liquid crystals in the form of a three dimensional network (Engineering Laboratory Report from Electric Information Institute, EID89-89, on page 1).
The complex of the liquid crystals and the transparent polymers are generally called as polymer dispersion type liquid crystals.
Typically, the method of producing the complex of the liquid crystals and the polymers includes the steps of: injecting a mixed composition, into which uncured resin monomers, such as ultraviolet curing acrylic or epoxy resins, and liquid crystal materials are dissolved, in between two substrates; and irradiating the mixed composition with ultraviolet to polymerize the resin monomers to produce a phase separation of the liquid crystals and the resins. This yields the structure of the liquid crystals being dispersed in the polymers or the structure of the polymer spreading in the liquid crystals in the form of network (Flat Panel Display ""91, on page 219, published by NIKKEI BP, and Engineering Laboratory Report By Electric Information Institute, EID89-89, on page 1, for example).
Some examples of this polymer dispersion type liquid crystal are disclosed by Japanese Laid-Open Patent Publications No. Hei 5(1993)-80302 and No. Hei 7(1995)-181454, in which in order to improve scattering effects of the complex of the liquid crystals and polymers with respect to light, the liquid crystal droplets are deformed or compressed to have a flat plate-like shape in section (i.e., the length vertical to the substrates is smaller than the length parallel with the substrates). Specifically, the Japanese Laid-Open Patent Publication No. Hei 5(1993)-80302 discloses that flat plate-like liquid crystal droplets are formed by being pressed under conditions of heating and that it is preferable that the flat liquid crystal droplets have a compression rate (deformation ratio) of 1.2 to 5.0 (which equates to 20 to 80 on a deformation rate basis, as mentioned later). The Japanese Laid-Open Patent Publication No. Hei 7(1995)-181454 discloses that flat plate-like liquid crystal droplets are formed by being pressed while being irradiated with ultraviolet and that it is preferable that the liquid crystal droplets are deformed to have a thickness of xc2xd of a length in section; in practice, the liquid crystal droplets are deformed at the rate of about xc2xd to xc2xc (which equates to 50 to 75 on a deformation rate basis, as mentioned later).
It is said that forming the liquid crystal droplets into flat plate-like shape provides advantages of increasing steepness and decreasing hysteresis.
Our study has proved, however, that when the liquid crystal droplets are deformed at the compression rate of 1.2 or more (which equates to 20 on a deformation rate basis, as mentioned later), as in the conventional examples mentioned above, deterioration in display characteristics such as reduction of contrast occurs, rather than improvement.
Further, according to Japanese Laid-Open Patent Publication No. Hei 5(1993)-80302, after the compressing process of the liquid crystals, the mixture is not subjected to any polymerization process. Due to this, the liquid crystal display element after compressed remain in the state in which polymerization reaction of the resin monomers immediately before compression is not yet completed and thus are incompletely cured. Hence, the compressing effects are relaxed with the passage of time, to cause the problem that the compressed liquid crystal droplets gradually return to their original before-compressed state. On the other hand, an attempt to prevent the relaxation of the compression effects after compression in the case where the mixture is not subjected to any polymerization process after the compressing process of the liquid crystals requires that the liquid crystal droplets are compressed in a state in which the polymerization of the resin monomers is completely concluded or is almost completely concluded. However, this disadvantageously needs much time for compression.
In this respect, according to Japanese Laid-Open Patent Publication No. Hei 7(1995)-181454, the mixture is polymerized after the compression of the liquid crystals, and as such solves the problem that the compressed liquid crystal droplets gradually return to their original before-compressed state as well as the problem that much time is needed for compression. However, with this prior art, since the two steps of polymerization are performed either by the photo polymerization with ultraviolet or by the thermal polymerization by heating, another problems are arisen as described below.
In the case where the two steps of polymerization are both performed by the photo polymerization with ultraviolet, even when the polymerization reaction of the resin monomers is desired to be, for example, 80% complete of the entire in the course of the polymerization before the compressing process, to allow for the compressing process of the liquid crystals, it is difficult to realize such a desired state, and variations in the degree of progress of the polymerization reaction may be caused for each of the liquid crystal cells. This is because the progress of the polymerization depends on the time for irradiation of ultraviolet as well as because the polymerization reaction can progress in a very short time. Thus, the variations in degree of progress of the polymerization reaction produce the variations in deformation ratio of the liquid crystals after the compressing process.
With this prior art, the degree of progress of the polymerization reaction has a strong influence on the compressing, particularly because the deformation ratio (compression rate) is very large. This causes the problem that the variations in deformation ratio caused by the variations in degree of progress of the polymerization reaction may exceed a tolerance to decrease the reliability of the liquid crystal display element. On the other hand, an attempt to increase the reliability of the liquid crystal display element results in reduction of manufacturing profits; increase of troublesome works; and an increase of manufacturing costs.
Further, in the case where the two steps of polymerization are both performed by the thermal polymerization, the polymerization reaction progresses slowly, so that variations in degree of progress of the polymerization reaction are not so greater than those by the photo polymerization, but nevertheless, in view of the deformation ratio being too great, the same problems as those in the case with the photo polymerization may be caused.
The polymer dispersion type liquid crystal display element, capable of eliminating the need for polarizers, have excellent characteristics of solving the problems of darkness, necessity of alignment phase treatment and difficulties in controlling cell gap, on the one hand, but have a disadvantage of possible deterioration in contrast on the other hand. Accordingly, the polymer dispersion type liquid crystal display element are being desired to have an improved contrast.
It is the key to improvement of contrast how the scattering of light should be produced while a voltage is not applied to electrodes between which the liquid crystals are sandwiched. It is now under discussion on how to increase cell gap or anisotropy xcex94n in dielectric constant in order to produce a full scattering of light.
Various approaches to increase the anisotropy xcex94n in dielectric constant have been proposed so far, but because of the limitations of liquid crystal materials and the like, none of them have succeed in providing satisfactory results.
With the approach to increase the cell gap, the number of scattering of light can be increased to an extent of the expansion of the cell gap, to ensure the scattering of light. However, the increase of the cell gap causes reduction of the electric field to be added to the liquid crystal layer for the application of the same voltage as that applied to an usual liquid crystal cell having a non-increased cell gap. Thus, an attempt to allow the liquid crystals to be oriented satisfactorily with the increased cell gap needs a larger voltage, which invites another problem of an increased voltage.
A polymer dispersion type liquid crystal display element capable of reducing a driving voltage, without any increase of the cell gap, while ensuring required contrast is disclosed in Japanese Laid-Open Patent Publication No. Hei 8(1996)-248398. In the prior art layers of polymer dispersion liquid crystal complex are laminated to form a laminated body and liquid crystal molecules in each layer of the polymer dispersion liquid crystal complex are aligned in a given direction in a plane substantially parallel with electrode surfaces. In addition, orientations of the liquid crystal molecular alignments in one layer of polymer dispersion liquid crystal complex and those in the other layer are so set as to intersect at right angles relative to each other by means of an alignment layer treatment such as a rubbing.
With this prior art of Japanese Laid-Open Patent Publication No. Hei 8(1996)-248398, the liquid crystal molecular alignments are regulated by means of the rubbing to improve the scattering effect, so only two layers of polymer dispersion liquid crystal complex can be formed. In addition, although the orientations of the liquid crystal molecules in one layer of polymer dispersion liquid crystal complex and those in the other layer are so set as to intersect each other at right angles, the liquid crystal molecules are oriented, in principle, only in two directions, when viewed from the cell gap direction, and hence it is difficult to obtain a fully satisfactory scattering of light. Besides, to begin with, the alignment treatment of the rubbing has a drawback of its requiring complex processes.
To sum up the problems of the prior art mentioned above,
{circle around (1)} When the liquid crystal droplets are deformed at a compression rate of 1.2 or more, as described in Japanese Laid-Open Patent Publications No. Hei 5(1993)-80302 and No. Hei 7(1995)-181454, deterioration in display characteristics, such as deterioration of contrast, occurs, rather than improvement.
{circle around (2)} An only single step polymerization, as described in Japanese Laid-Open Patent Publication No. Hei 5(1993)-80302, involves a problem that the compressing effects are relaxed with the passage of time, so that the compressed liquid crystal droplets gradually return to their original before-compressed state; or a problem that more time is needed for the compression.
Also, when the polymerization is performed in two steps either by the photo polymerization with ultraviolet or by the thermal polymerization by heating, as described in Japanese Laid-Open Patent Publication No. Hei 7(1995)-181454, the problem of decrease in reliability of the liquid crystal element is caused, on the one hand, and an attempt to increase the reliability of the liquid crystal element leads to problems of reduction of manufacturing profits, increase of troublesome works and increase of manufacturing costs, on the other hand.
{circle around (3)} When the liquid crystal molecular alignments are regulated by means of the rubbing to improve the scattering effect, as described in Japanese Laid-Open Patent Publication No. Hei 8(1996)-248398, it is difficult to obtain a fully satisfactory scattering of light. Also, to begin with, the alignment layer treatment of the rubbing has a drawback of its requiring complex processes.
It is accordingly an object of the present invention to provide a polymer dispersion type liquid crystal display element capable of providing improved scattering characteristics without any deterioration of display characteristics, and a producing method thereof.
Another object of the invention is to provide a polymer dispersion type liquid crystal display element capable of providing an increased steepness of the transmitted light amount with respect to voltage, to enable a passive matrix drive, and a producing method thereof.
Still another object of the invention is to provide a method for producing a liquid crystal display element which maintains compressed effects of liquid crystals for a long time to produce an enhanced reliability and an apparatus for use in the producing method.
A further object of the invention is to provide a liquid crystal display element capable of providing satisfactory scattering effects without any rubbing treatment and without any increase of a driving voltage and a producing method thereof.
The above objects are accomplished by a polymer dispersion type liquid crystal display element comprising a pair of substrates; an electrode formed on an opposite inner surface of each of the substrates; polymers; liquid crystal droplets; and polymer liquid crystal complex held between the pair of substrates, the polymer liquid crystal complex comprising the polymers and the liquid crystal droplets dispersed in the polymers, and liquid crystal molecules in the liquid crystal droplets being oriented in a direction parallel to the substrates and oriented randomly in a plane parallel to the substrates.
Also, the above objects are accomplished by a polymer dispersion type liquid crystal display element comprising a pair of substrates; an electrode formed on an opposite surface of each of the substrates; polymers; liquid crystal droplets, the liquid crystal droplets being deformed into a compressed structure of their being contracted in a cell gap direction; and polymer liquid crystal complex held between the pair of substrates, the polymer liquid crystal complex comprising the polymers and the liquid crystal droplets dispersed in the polymers, an amount of deformation of the liquid crystal droplets being set to be in range in which a phenomenon of liquid crystal molecules rising up in the cell gap direction is not caused by excluded volume effects of the liquid crystals.
It is noted here that the term xe2x80x9cpolymer dispersion type liquid crystal elementxe2x80x9d is intended to include polymer dispersion type liquid crystal element generally known as a complex of polymers and liquid crystals, including NCAP (Nematic Curvilinear Aligned Phase), PDLC (Polymer Disperse Liquid Crystal) and PNLC (Polymer Network Liquid Crystal). Hence, the term xe2x80x9cpolymer dispersion type liquid crystal elementxe2x80x9d as used herein is intended to include not only the one in which liquid crystal droplets are dispersed in an island form in a polymer matrix or are associated in series but also the one in which resins spread in continuous phases of liquid crystals in the form of a three dimensional network.
In addition, the term xe2x80x9camount of deformation of liquid crystal dropletsxe2x80x9d as used herein is intended to mean the ratio of xe2x80x9ca difference between the length of a liquid crystal droplet extending along the direction parallel to the substrates and the length thereof extending along the call gap directionxe2x80x9d relative to xe2x80x9cthe length of the liquid crystal droplets extending along the direction parallel to the substratesxe2x80x9d.
Following is the reason the construction of the invention can accomplish the object of improving scattering effects.
With a typical polymer dispersion type liquid crystal display element, liquid crystal droplets have a spherical form and the liquid crystal droplets are then oriented on a random basis not only with respect to the substrates but also in a plane parallel to the substrates. This random alignment of the liquid crystal droplets is caused by the spherical liquid crystal droplets having a symmetry property and lacking of regularity in the direction for poles to be produced. This alignment of the general type of liquid crystals cannot fully provide the scattering effects. Accordingly, it is thought that it is effective for obtaining the satisfactory scattering effects to treat the liquid crystal molecules in the liquid crystal droplets to be aligned in the direction parallel to the substrates. This is because since the scattering of light is produced by mismatch in difference in refractive index between the liquid crystals and the polymers and the mismatch in refractive index between the liquid crystal droplets, aligning the liquid crystals in the horizontal direction to the substrates allows effective refractive index anisotropy xcex94n to be increased, and as a result of this, the scattering of light is increased. However, even when the liquid crystal molecules are aligned in parallel to the substrates, if the liquid crystal molecules are oriented in a uniform direction, the scattering of light is reduced. This is because since the difference in refractive index between the liquid crystal droplets is small, a sufficient intensity of the scattering is not obtained. Hence, the most desirable formation of the liquid crystal molecular alignments for improving the scattering effects is that the liquid crystal molecules in the liquid crystal droplets are aligned in parallel to the substrates and also are oriented randomly in a plane parallel to the substrates.
On the other hand, forming the liquid crystal droplets into a compressed shape enables the liquid crystal molecules to be aligned in parallel to the substrates. This is because when the liquid crystal droplets are deformed into a structure of their being compressed in the cell gap direction, the length of each liquid crystal droplet extending along the cell gap direction becomes smaller than the length thereof extending along the direction parallel to the substrates, so that the liquid crystal droplets come to have asymmetry. It was found that the liquid crystal molecules in the liquid crystal droplets thus compressed were oriented in the direction parallel to the substrates. Accordingly, the liquid crystals are so oriented that bipolar axes come close to parallel to the substrates. However, the liquid crystal droplets compressed in the cell gap direction still have a circular shape in cross section taken along a line parallel to the substrates without deformation. Thus, the liquid crystal droplets have symmetry in a plane parallel to the substrates, and as such provides no regularity for the liquid crystal molecular alignments and allows the same to be oriented randomly. Thus, deforming the liquid crystal droplets into a compressed form enables the liquid crystal molecules in the liquid crystal droplets to be oriented in parallel to the substrates and also be oriented randomly in a plane parallel to the substrates, thus providing improved scattering effects.
It should be noted here that the improvement of the scattering effects is provided by the liquid crystals being aligned in parallel to the substrates by the deformation and also being oriented randomly in a plane parallel to the substrates, rather than directly by the deformation of the liquid crystal droplets. Hence, irrespective of the amount of deformation of the liquid crystal droplets, any deformation of the liquid crystals does not always provide improved scattering effects. In this respect, it was confirmed by the inventors that an excessive deformation produces deterioration of characteristics, rather than improvement. The reason therefor is that with an increasing deformation of the liquid crystal droplets, a tendency of the liquid crystal molecules in the liquid crystal droplets being aligned in the cell gap direction (the vertical direction to the substrates) is increased by excluded volume effects of the liquid crystals; in other words, the excluded volume effects of the liquid crystals cause the liquid crystal molecules to rise up vertically.
According to the invention, the liquid crystal droplets are deformed within the range in which any force which may cause the liquid crystal molecules to rise up is not produced by the excluded volume effects of the liquid crystals, whereby the orientations of the liquid crystal molecular alignments in the liquid crystal droplets can come relatively dose to parallel to the substrates. As a result of this, the polymer dispersion type liquid crystal display element capable of providing improved scattering characteristics without any deterioration of display characteristics, such as contrast, can be attained.
Experiments were conducted by the inventors in both conditions of a normal anchoring strength and of a strong anchoring strength, the results showing that when the amount of deformation of the liquid crystal droplets exceeds 10% in the normal anchoring strength, the phenomenon of the liquid crystal molecules rising up vertically is caused by the excluded volume effects of the liquid crystals. Hence, when the amount of deformation of the liquid crystal droplets is of 10% or less in the normal anchoring strength, it is ensured that the orientations of the liquid crystal molecular alignments in the liquid crystal droplets can come relatively close to parallel to the substrates. Also, when the amount of deformation of the liquid crystal droplets exceeds 20% in the strong anchoring strength, the phenomenon of the liquid crystal molecules rising up vertically is caused by the excluded volume effects of the liquid crystals. Hence, when the amount of deformation of the liquid crystal droplets is of 20% or less in the strong anchoring strength, it is ensured that the orientations of the liquid crystals molecular alignments in the liquid crystal droplets can come relatively close to parallel to the substrates.
According to the invention, the amount of deformation of the liquid crystal droplets may be expressed by parameters of a mean value xcex8p of the angles formed by the liquid crystal molecules and the substrates; a dielectric ratio E; or a deformation rate P. This is because the tilt angle of a liquid crystal molecule with respect to a substrate, the dielectric ratio E and the deformation rate P each vary with the deformation of the liquid crystal droplets.
According to the invention, a pressing through a vacuum package, an application of a hydrostatic pressure and the like may be used as means for deforming the liquid crystals.
A liquid crystal display element according to the invention may be formed such that the liquid crystals are not be deformed in the cell gap direction, and the mean value xcex8p of the angles formed by the liquid crystal molecules and the substrates can be lowered. This liquid crystal display element can be produced by applying magnetic field across the substrates in the course of the liquid crystals being separated out or by giving polarization to ultraviolet.
The liquid crystal display element according to the invention can raise steepness, so that the invention can suitably work not only to the liquid crystal display element of active matrix drive but also to that of the passive matrix drive.
A liquid crystal display element according to the invention may be formed such that a complex of polymers and the liquid crystals is held by a pair of substrates; the polymers are obtained by liquid crystal monomers employed as polymerizable monomers being subjected to photo polymerization; azimuths angles of the liquid crystals in a plane of the substrates are random; and the means value xcex8p of the angles formed by the liquid crystal molecules and the substrates are lowered. This liquid crystal display element is obtained by liquid crystal monomers employed as polymerizable monomers being polymerized through the irradiation of ultraviolet to allow the liquid crystals to be separated out.
When the liquid crystal monomers are employed for the polymerizable monomers, the mixture of the liquid crystals and the polymerizable monomers exhibits a homogeneous liquid crystal layer. This acts to allow the liquid crystals to be separated out without any impairment of the liquid crystal layer when the liquid crystals are separated out by polymerization of the monomers. Hence, if the molecules of the mixture exhibiting the liquid crystal layer before polymerization are conditioned so that they can be aligned in parallel to a plane of the substrates and also oriented in random in a plane of the substrates, alignments of the liquid crystal molecules separated out by the irradiation of ultraviolet will be affected by the condition before polymerization, with the result that the liquid crystal molecules after polymerization will be aligned nearly in parallel to the plane of the substrates and also oriented in random in the plane of the substrates.
To accomplish the above objects, a method of producing liquid crystal display element according to the invention comprising two steps of polymerization of a first polymerization process and a second polymerization process, and a deformation treatment of the liquid crystal molecules is performed after the first polymerization process, followed by performing the second polymerization process, characterized by that the first polymerization is performed by thermal polymerization and the second polymerization is performed by photo polymerization, a thermal polymerization initiators"" content in the mixture being set to such an amount that when the thermal polymerization initiators are wholly consumed by thermal polymerization reactions, the polymerizable materials are brought into a first predetermined polymerization state and a photo polymerization initiators"" content being set to such an amount that when the photo polymerization initiators are wholly consumed by at least photo polymerization reactions, the polymerizable materials can be polymerized from the first polymerization state until a second polymerization state in which the polymerization reaction of all the polymerizable materials is completely concluded.
With the above construction, the thermal polymerization initiators are reacted in the first polymerization process and all thermal polymerization initiators are consumed, to produce the first polymerization state. In this state, for example more than half the polymerizable materials or more are polymerized, so that the mesh sizes of the complex of the liquid crystals and the polymers are almost determined. Then, the liquid crystals are pressed and are each formed into a compressed form in a cell gap direction. In the next second polymerization process, the remaining unreacted polymerizable materials are completely polymerized by reaction with the photo polymerization initiators existing in the complex to cause cross-link. This allows the compressed form of the liquid crystals to be fixed and stabilized. According to this constructed invention, since the first polymerization process is performed by the thermal polymerization and the second polymerization process is performed by the photo polymerization process, a desired first polymerization state can be obtained by adjusting the thermal polymerization initiators"" content. Hence, the abovesaid problems involved in the prior art in which the first and second polymerization processes are both performed by the photo polymerization can be solved.
Further, the invention may be constructed such that the first polymerization is performed by the photo polymerization and the second polymerization is performed by the thermal polymerization; a photo polymerization initiators"" content in the mixture is set to such an amount that when the photo polymerization initiators in the mixture are wholly consumed by photo polymerization reactions, the polymerizable materials are brought into a first predetermined polymerization state; and a thermal polymerization initiators"" content is set to such an amount that when the thermal polymerization initiators are wholly consumed by at least thermal polymerization reactions, the polymerizable materials can be polymerized from the first polymerization state until a second polymerization state in which the polymerization reaction of all the polymerizable materials is completely concluded.
With the above construction, the photo polymerization initiators are reacted in the first polymerization process and all photo polymerization initiators are consumed, to produce the first polymerization state. In this state, for example more than half the polymerizable materials or more are polymerized, so that the mesh sizes of the complex of the liquid crystals and the polymers are almost determined. Then, the liquid crystals are pressed and are each formed into a compressed form in a cell gap direction. In the next second polymerization process, the remaining unreacted polymerizable materials are completely polymerized by reaction with the thermal polymerization initiators existing in the complex to cause cross-link. This allows the compressed form of the liquid crystals to be fixed and stabilized.
According to this constructed invention, since the first polymerization process is performed by the photo polymerization and the second polymerization process is performed by the thermal polymerization process, a desired first polymerization state can be obtained by adjusting the photo polymerization initiators"" content. Hence, the abovesaid problems involved in the prior art in which the first and second polymerization processes are both performed by the photo polymerization can be solved.
Further, the invention may be constructed such that one of the substrates is provided on its surface with a metal wiring of a stripe or matrix form; the mixture is irradiated with ultraviolet from the one substrate side of the liquid crystal cell in which the mixture is injected, to allow the polymers and the liquid crystals to be phase separated by the photo polymerization; and after the extrusion process, the mixture is irradiated with ultraviolet from the other substrate side, to allow the remaining unreacted polymerizable materials to be polymerized by the photo polymerization.
With the above construction, an area of the substrate on which the metal wiring of strip or matrix form is provided is shielded from ultraviolet by the metal wiring. Thus, the ultraviolet, when irradiated from the one substrate side, can only pass through apertures (parts other than the metal wiring parts) of the one substrate, and the mesh sizes of the complex of the liquid crystals and polymers are determined by polymerization reaction of the mixture existing in the areas through which the ultraviolet can pass. However, the mixture existing in areas under the metal wiring is only partially polymerized by a leaked light and is almost unreacted. When the substrates are pressed to be deformed in a cell gap direction, the liquid crystals at the apertures can be moved along the metal wiring parts of strive or matrix form to be easily squeezed out from the end-sealing part, thus enabling the liquid crystals to be formed into a compressed form very easily. Thereafter, when the ultraviolet is irradiated from the other substrate side, the mixture existing in the areas at the metal wiring parts is polymerized, and the polymerized materials play a role of an adhesive for the entire substrate to fix and stabilize the compressed form of the liquid crystal droplets.
The one substrate may be an active substrate on which a pixel electrode and an active element are formed for each of a plurality of regions partitioned by the wiring of a matrix form. Also, the active substrate may be TFT.
Also, where a glass-transition temperature of the polymers in the first polymerization state is set Tg1 and a glass-transition temperature of the polymers in the second polymerization state is set Tg2, Tg2 may be made higher than the Tg1 by 10xc2x0 C. or more. This enables the compression of the liquid crystals to be facilitated, because the liquid crystals before extrusion are low in glass-transition temperature and soft. Further, since the liquid crystals after the second polymerization process is high in glass-transition temperature, the resins become so hard that the tendency of liquid crystals to return to the before-compression state can be suppressed.
Further, the extrusion of the liquid crystals may be performed by heating. Since a coefficient of thermal expansion of the liquid crystal materials is much larger than that of the polymers and that of the cell substrates, the heating allows the liquid crystals to be squeezed out from the end-sealing part, without using any pressing apparatus, to form the liquid crystals into a compressed form and also allows the compressed form of the liquid crystals to be fixed and stabilized by returning the raised temperature to a category temperature range of 60xc2x0 C. or less after the end-sealing part is sealed by sealing resins.
Other suitable pressing means, such as a press and a vacuum package, may be used for extruding the liquid crystals.
Additionally, the invention may be constructed such that the fist and second polymerization are both performed by the photo polymerization; the liquid crystal cell is cooled so that the liquid crystal droplets separated out in the first polymerization process can be deformed in a cell gap direction; and after the cooling process, the mixture is irradiated with ultraviolet in the second polymerization process, with the liquid crystal cell kept in its cooled state, so that the remaining unreacted polymerizable materials can be polymerized by the photo polymerization.
With the above construction, the photo polymerization initiators are reacted in the first polymerization process and for example more than half the polymerizable materials or more are polymerized, so that the mesh sizes of the complex of the liquid crystals and the polymers are almost determined. Then, the liquid crystal cell is cooled to deform the liquid crystals into a compressed form contracted in a cell gap direction by pressing based on a thermal stress resulting from a lager coefficient of thermal expansion of the liquid crystal materials. In this state, the polymerizable materials are irradiated with ultraviolet in the second polymerization process, so that the remaining unreacted polymerizable materials are completely polymerized by reaction with the photo polymerization initiators existing in the complex to fix and stabilize the compressed form of the liquid crystals.
In addition, the invention may be constructed such that the first polymerization process is performed by the thermal polymerization; the second polymerization process is performed by the photo polymerization; the liquid crystal cell is cooled so that the liquid crystal droplets separated out in the first polymerization process can be deformed in the cell gap direction; and after the cooling process, the mixture is irradiated with ultraviolet in the second polymerization process, with the liquid crystal cell kept in its cooled state, so that the remaining unreacted polymerizable materials can be polymerized by the photo polymerization.
With the above construction, the thermal polymerization initiators are reacted and are all consumed in the first polymerization process to produce the first polymerization state. In this state, for example more than half the polymerizable materials or more are polymerized, so that the mesh sizes of the complex of the liquid crystals and the polymers are almost determined. Then, the liquid crystal cell is cooled to deform the liquid crystals into a compressed form contracted in the cell gap direction by pressing based on a thermal stress resulting from a larger coefficient of thermal expansion of the liquid crystal materials. In this state, the mixture is irradiated with ultraviolet in the second polymerization process, so that the remaining unreacted polymerizable materials are completely polymerized by reaction with the photo polymerization initiators existing in the complex to fix and stabilize the compressed form of the liquid crystals. According to this invention, since the first polymerization process is performed by the thermal polymerization and the second polarization process is performed by the photo polymerization process, the abovesaid problems involved in the prior art in which the first and second polymerization processes are both performed by the photo polymerization can be solved by adjusting the thermal polymerization initiators"" content.
Also, the invention is directed to a production apparatus comprising a means for carrying the liquid crystal cell filled with the mixture along a carriage path; an ultraviolet irradiation means, arranged at a midpoint of the carriage path, for radiating ultraviolet to a part of the liquid crystal cell; and pressing means, arranged at a midpoint of the carriage path, for pressing the area of the liquid crystal cell irradiated with ultraviolet.
The above construction can provide the result that the first polymerization process and the extrusion process of the liquid crystals can be performed automatically. In the case where the first and second polymerization processes are both performed by the photo polymerization, an attempt to control the polymerization reaction by merely regulating the ultraviolet irradiation time may often have difficulties in forming the separated liquid crystals in a compressed form, because the polymerization reaction progresses for a certain period alter the irradiation of ultraviolet. According to the invention, controlling a rotation speed of rollers and a feed rate of the liquid crystal cell to be optimum enables the liquid crystal cell to be pressed at a point the liquid crystal cell becomes an optimal polymerization state during the period from the initiation of polymerization by the irradiation of ultraviolet until the conclusion of polymerization in which phase separation of the liquid crystals by the polymerization is completed. As a result, forming the liquid crystals into a compressed form can be facilitated. Even if the polymerization by the irradiation with ultraviolet is performed only once and no polymerization process is performed after the extrusion process of the liquid crystals, since the liquid crystal cell can be pressed with the rollers at a point the liquid crystal cell becomes an optimal polymerization state, allowing for the compression of liquid crystals and relaxation of the compression effects after compression by controlling the rotation speed of rollers and the feed rate of the liquid crystal cell to be optimum in substantially the same manner as in the above, the liquid crystal display element relatively stable in compression effects after compression can be produced.
Further, in the production apparatus according to the invention, the ultraviolet irradiation means may comprise a light source for emitting ultraviolet and an optical filter to intercept only wavelength components which allow the liquid crystals to be optically resolved, and it may further include a temperature control mechanism to maintain the liquid crystal cell at a preset temperature in association with the phase-separation reaction of the liquid crystals by irradiation of ultraviolet. This construction enables the phase-separation reaction of the liquid crystals to be controlled sensitively to temperature, thus obtaining the liquid crystal display element having homogeneous compression effects. Further, the optical filter arranged enables the liquid crystals to be prevented from being optically resolved by ultraviolet, thus providing improved reliability of the liquid crystal display element.
Further, according to the invention, when the liquid crystal element is produced by the two steps of polymerization processes, the liquid crystal droplets may be deformed at the deformation ratio of 1.15 or less in the deformation process. It is noted here that the term xe2x80x9cdeformation ratioxe2x80x9d used herein is intended to mean xe2x80x9cthe before-deformation to after-deformation ratio of the cell gapxe2x80x9d. The reason for making the deformation ratio 1.15 or less is that an excessive deformation provides deterioration of the scattering of light, rather than improvement. This deformation rate made much smaller than that of the prior art can provide improved scattering of light and can also provide specific effects mentioned below for the invention according to which the first and second polymerization processes are both performed by the photo polymerization. That is to say, as mentioned at the xe2x80x9cDescription of the Prior Artxe2x80x9d, in the case where the first and second polymerization processes are both performed by the photo polymerization, variations in deformation ratio are caused by variations in the degree of progress of the polymerization reaction in the first polymerization process. However, with the invention according to which the deformation ratio is much smaller than that of the prior art, the deformation ratio is little affected by variations in the degree of progress of the polymerization reaction, so that poor reliability of liquid crystal display element, which is the problem involved in the prior art, can be solved.
Further, to accomplish the above objects, polymer dispersion type liquid crystal display element according to the invention have a structure in which a plurality of polymer liquid crystal complex layers in which liquid crystal droplets are dispersed in polymers are laminated to form a layered product and the layered product is held between a pair of substrates on which electrodes are formed, characterized by that liquid crystal molecules in the liquid crystal droplets in each of the polymer liquid crystal complex layers are aligned in the direction substantially parallel with the substrates and also are oriented in different directions in a plane parallel to the substrates for each of the polymer liquid crystal complex layers.
With the above construction the liquid crystal molecules in the liquid crystal droplets in each of the polymer liquid crystal complex layers are aligned in the direction substantially parallel with the substrates and also are oriented nearly randomly as a whole when the liquid crystal display element is viewed along the cell gap direction. This enables the difference in refractive index between the respective layers to be considerably increased, as compared with the prior art, to provide satisfactory scattering effects. Also, each of the layers is reduced in thickness to form a laminated structure with the number of laminated layers, so as to provide satisfactory scattering effects. Due to this, there is no need to increase the thickness of the cell, so an increase in driving voltage can be avoided.
To obtain the above liquid crystal display element, in the polymerization process in which the mixture is irradiated with ultraviolet to allow the polymers and the liquid crystals to be phase separated to form the polymer liquid crystal complex layers, a polarized ultraviolet polarized in one direction may be used as the ultraviolet and the polarizing direction of the polarized ultraviolet may be changed for each of the polymer liquid crystal complex layers to control the liquid crystal alignments.
Alternatively, in the course of the polymerization process in which the mixture is irradiated with ultraviolet to allow the polymers and the liquid crystals to be phase separated to form the polymer liquid crystal complex layers, magnetic field or electric field may be added in a different direction for each of the polymer liquid crystal complex layers to control the liquid crystal alignments. These producing methods of the invention can provide liquid crystal display element of high contrast with simplified manufacturing process, without using any rubbing as the prior art.
Further, the liquid crystal display element according to the invention may be designed such that the liquid crystal molecules in the liquid crystal droplets in the polymer liquid crystal complex layers are aligned in the direction substantially parallel to the substrates and also are oriented randomly in a plane substantially parallel to the substrates. This arranged liquid crystal display element is the most preferable in alignments of the liquid crystals.
In obtaining the above liquid crystal display element, the substrates are pressed after the polymer liquid crystal complex layers are formed between the substrates by the polymerization process, so that the alignment of the liquid crystals are controlled. In detail, when the polymerizable materials are polymerized to allow the polymers and the liquid crystals to be phase separated, to form the polymer liquid crystal complex layers, the liquid crystal molecules in the liquid crystal droplets in the polymer liquid crystal complex layers thus produced are oriented randomly not only with respect to the substrates but also in a plane parallel to the substrates. It is known that when the substrates are pressed in this state, to form the liquid crystal droplets into a compressed form, the liquid crystal molecules in the liquid crystal droplets are oriented in the direction generally parallel to the substrates. Thus, by pressing the substrates, the liquid crystal molecules in the liquid crystal droplets in the polymer liquid crystal complex layers can be so controlled as to be oriented in the direction substantially parallel to the substrates and also be oriented randomly in a plane substantially parallel to the substrates.
The liquid crystal droplets may be formed with coloring matters of dichroism being mixed with the liquid crystals. This can produce a guest host type liquid crystal display element capable of absorbing light by means of the coloring matters of dichroism.